Globular Clusters as Million-Body Gigayear Relics

Globular clusters are real-life realizations of large-N particles interacting gravitationally. These objects are almost as old as the universe, containing populations older than ten gigayears. Their negative heat capacities mean that star clusters tend to have runaways in their central stellar densities over time, and the relaxation timescales of globular clusters place globular clusters at exactly the right age regime to observe so-called core-collapsed clusters.

The advent of Hénon-style Monte Carlo codes for solving star cluster dynamics (in particular, Cluster Monte Carlo) allowed for the integration of globular-like clusters under realistic stellar evolution and few-body dynamical models in scientifically useful times.

Dynamical observables such as a cluster’s surface brightness profile or velocity dispersion profile can help us match a present-day globular cluster to a cluster model. This allows us to query a grid of models to match a real globular cluster to a set of “best-fitting” models, which gives us the ability to peer into the theoretical stellar populations (and, in particular, black hole populations) of globular clusters.

Globular clusters are an incredibly vibrant field of research and have been reviewed countless times. An incredibly inspiring is review of star cluster dynamics given by The Gravitational Million-Body Problem by Douglas Heggie and Piet Hut.

Slides for the talk I gave for an Illinois Space Grant seminar can be found here. This work was conducted at the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) at Northwestern University under the Illinois Space Grant with Fred Rasio and Kyle Kremer.

Solid-State Defects as Nanoscale Sensors

Nitrogen-vacancy (NV) centers are solid-state defects in diamond with useful optical behavior and couplings to the environment. These properties allow NV centers to be used as nanometer-scale sensors of electromagnetic and strain fields. NV centers are thus an extremely promising tool for probing individual charges within a diamond and measuring magnetism and strain with unprecedented resolution, even in high-pressure applications where NV centers can be created directly within diamond anvil cells.

Lilian Childress’s thesis serves as a comprehensive review of nitrogen vacancy centers.

Christian Hepp’s thesis reviews another interesting solid-state defect, the silicon-vacancy center. The lack of a first-order susceptibility makes it an especially promising candidate for strain sensing applications.

This work was conducted by Norman Yao’s group. I am gracious for the mentorship of Satcher Hsieh as well as many others.

NV centers push the frontiers of high-pressure magnetometry to unprecedented sensitivities and spatial resolutions.

Dynamical Structure of the Quintuplet Cluster

The Quintuplet cluster is a young massive cluster which is extremely close to the Galactic Center. It is an intriguing example of star formation and stellar dynamics in an extreme tidal environment.

Proper motions enable the identification of cluster members in the face of extreme crowding and differential reddening. We examined the structure of the Quintuplet cluster using data from the WFC3-IR instrument on the Hubble Space Telescope, finding marginal evidence of mass segregation, no evidence for asymmetry or a tidal truncation, and a core radius significantly larger than that of the Arches.

The latter observation hints at significant evolution of star clusters in the Galactic Center on megayear timescales, suggesting the validity of predictions that such clusters dissolve in around ten megayears.

Simon Portegies Zwart’s classic review of young massive clusters can be found here.

Slides for the Departmental Lunch Talk I gave on 21 March 2019 can be found here. Our proper motion study of the Quintuplet can be found here. This work was conducted with Professor Jessica Lu (UCB) and Matt Hosek (UCLA).

Three-color image of the Quintuplet cluster, where the RGB values represent intensities in three NIR filters on HST WFC3-IR (120”×120”).